Recent spread of mobile electronic devices such as mobile video cameras and mobile PCs has rapidly increased demand for secondary batteries as portable power sources. A demand for downsizing, weight reduction and higher energy density of such secondary batteries is strong.
Conventionally, secondary batteries capable of being repeatedly charged/discharged are mostly lead batteries, nickel-cadmium batteries, and the like. Such batteries have excellent charge/discharge characteristics but are not enough as portable power sources for mobile electronic devices in terms of the weight or energy density thereof.
To solve the above problem, lithium secondary batteries including a lithium or lithium alloy anode are now studied and developed as secondary batteries. Such lithium secondary batteries advantageously have a high energy density, less self-discharge, and a light weight.
Electrodes of lithium secondary batteries are formed commonly by kneading an active material and a binder together with a solvent to give a slurry containing the active material dispersed therein, applying the slurry onto a current collector by the doctor blade method or the like, and drying the applied slurry into a thin film.
At the present, fluororesins typified by polyvinylidene fluoride (PVDF) are most widely used as binders for electrodes (anodes) of lithium secondary batteries.
Polyvinylidene fluoride is however less likely to be dissolved in a solvent upon production of a slurry for an electrode, which significantly lowers the production efficiency.
To solve this problem, N-methylpyrrolidone, which dissolves polyvinylidene fluoride therein, is commonly used as a solvent for slurry. N-methylpyrrolidone however has a high boiling point, which requires a large amount of heat energy in the step of drying the slurry. In addition, undried N-methylpyrrolidone may be left in the electrode to lower the battery performance.
An aqueous slurry containing water as a solvent is now considered to be used as a slurry for electrodes including cathodes and anodes. In this method, carboxymethyl cellulose is commonly used as a water-soluble binder resin.
In the case of using carboxymethyl cellulose, however, an ability of binding the active material or adhesion to a current collector is markedly poor due to insufficient flexibility of the resin.
Patent Literature 1 discloses a method of dispersing styrene/butadiene latex as a binder. In this case, however, since an excessive amount of an organic component is added, the amount of the active material is relatively reduced to lower the capacity of the battery.